Because enzymes can catalyze chemical transformations so effectively, there is increasing emphasis on the use of enzyme reactions in commercial processes, and their relatively high cost demands reuse. Typically, if the reaction is performed under homogeneous conditions recovery of the enzyme is difficult and expensive, which effectively precludes homogeneous enzymatic catalysis. The solution to this problem is to insolubilize the enzyme under conditions where a substantial portion of the enzymatic activity exhibited in solution remains under heterogenous reaction conditions.
One particular solution to the aforementioned problem is the construction of immobilized enzyme systems. An immobilized enzyme system consists of a support matrix to which there is bound an enzyme. A support matrix is a structure characterized as having good physical integrity and favorable properties toward liquid flow under conditions experienced in fixed bed reactors, and further characterized by having the ability to bind or immobilize enzymes with minimum perturbation of enzymatic action. By an immobilized enzyme system is meant the structure which results from immobilization of an enzyme on a support matrix.
The binding or immobilization of enzymes to a support matrix is represented by the extremes of physical and chemical binding forces. It is to be recognized that in most cases enzyme immobilization arises from a combination of such binding forces, although often one such force predominates, with the nature of enzyme immobilization generally being determined by the nature of the support matrix. As an example, when the support matrix is a resin, such as one of the phenol-formaldehyde type, binding is predominantly through physical forces. A similar result is obtained when the support matrix is of an ion exchange type. Where the support matrix is comprised of refractory inorganic material, such as inorganic oxides, glass, and ceramics, bearing or impregnated with organic material, for example, polyamines, either bearing pendant functional groups themselves or cross-linked with a bifunctional reagent which provides pendant functional groups, enzyme immobilization arises mainly by chemical reaction of a site on the enzyme with the pendant functional group so as to form a covalent bond. In such an instance binding is, at least predominantly, by chemical means.
An advantage of immobilized enzyme systems where the enzyme is chemically bound to the support matrix is an increased resistance to enzyme loss, as manifested by an increased half-life. In immobilized enzyme systems of the polyamine type cross-linked by excess gluteraldehyde so as to furnish pendant aldehyde groups, as described in U.S. Pat. No. 4,141,857, binding occurs by reaction of the aldehyde group with an amino group from the enzyme. The resultant imine bond, --CH.dbd.N--, is susceptible to hydrolysis, which presents a limitation upon the half-life of such covalently bound enzymes.
An object of this invention is to supply an immobilized enzyme system where the enzyme is covalently bound to the support matrix by a hydrolytically stable linkage. An embodiment is a support matrix comprising a porous support impregnated with a polyamine substantially all of whose nitrogens bear a pendant epoxide group. In a more specific embodiment the polyamine is polyethyleneimine. In a still more specific embodiment the pendant epoxide groups are 2,3-epoxypropyl groups.